The present invention generally relates to extracting invisible information, more specifically, relates to a system, a method, and a program for extracting invisible information even if visible information overlaps with the invisible information.
Methods for combining analog media (information) such as paper and digital contents (information) include the following method: first, information such as URL is encoded in a QR code (two-dimensional barcode) or the like; secondly, the QR code is printed in an invisible state on paper in a stealth ink or the like; thirdly, it is imaged by a digital camera or a camera phone under irradiation of light (ultraviolet light or the like) that makes the stealth ink luminous to thereby extract the encoded information (Japanese Unexamined Patent Application Publication No. 2003-296659).
The known method has the advantages of having a good appearance and saving space as compared with a method of printing the QR code or the like in visible ink. This method also has the advantages in embedding overwhelming amount of information in comparison with a method of embedding information by digital watermarking, having high resistance, and not degrading image quality at all.
This method, however, has the problem that when the invisible code and a visible print are printed double, correct invisible information cannot be extracted. In other words, although the QR code has the function of correcting burst errors of a maximum of 30 percent with a Reed-Solomon code, overlapped printing of the invisible code and a visible print may cause an error beyond the error correction capability.
In order to solve the problem, there is a method of printing so as to prevent such overlap as much as possible (Japanese Patent Application No. 2005-044666). However, overlapped printing is sometimes unavoidable in terms of space and so on.